\documentclass{exam-zh}
\examsetup{
  solution = {
    show-solution = true,
    parbreak      = true
  }
}

\begin{document}

\begin{problem}
  下面各题，能简算的要简算。	
  \begin{choices}[label={}, columns=3]
    \item 
      \begin{minipage}[t]{0.25\textwidth}
        $600-300 \div 20 \times 5 $
        \begin{solution}
            \begin{minipage}{6em}
              \begin{align*}
                \text{原式}&	= 600 - 15 \times 5\\
                & = 600 - 75\\
                & = 525  \qedhere
              \end{align*}
            \end{minipage}
        \end{solution}
      \end{minipage}
    
    \item 
      \begin{minipage}[t]{0.25\textwidth}
        $ (4.8-3.8 \div 2) \times 0.3 $
        \begin{solution}
          \begin{minipage}{6em}
            \begin{align*}
              \text{原式}&	= (4.8 - 1.9) \times 0.3\\
              & = 2.9 \times 0.3\\
              & = 0.87 \qedhere
            \end{align*}
          \end{minipage}
        \end{solution}
      \end{minipage}
    
    \item 
      \begin{minipage}[t]{0.25\textwidth}
        $3.45 \times 99 \times 2+6.9 $
        \begin{solution}
          \begin{minipage}{6em}
            \begin{align*}
              \text{原式}&	= 6.9 \times 99 + 6.9\\
              & = 6.9 \times (99 + 1)\\
              & = 6.9 \times 100 \\ 
              & = 690 \qedhere
            \end{align*}
          \end{minipage}
        \end{solution}	
      \end{minipage}
    \item 
      \begin{minipage}[t]{0.25\textwidth}
        $\left(8 + \dfrac{4}{9}\right) \div 4$
        \begin{solution}
          \begin{minipage}{6em}
            \begin{align*}
              \text{原式}&	= 8 \div 2 + \frac{4}{9} \div 2 \\
              & = 2 + \frac{2}{9}\\
              & = 2 \frac{2}{9}  \qedhere	    	
            \end{align*}
          \end{minipage}
        \end{solution}	
      \end{minipage}
    \item 
      \begin{minipage}[t]{0.25\textwidth}
        $\dfrac{8}{13} \div 9 + \dfrac{1}{9} \times \dfrac{1}{13}$
        \begin{solution}
          \begin{minipage}{6em}
            \begin{align*}
              \text{原式}&	= \frac{8}{13} \times \frac{1}{9} + \frac{1}{13} \times \frac{1}{9}  \\
              & = (\frac{8}{13} + \frac{1}{13})  \times \frac{1}{9} \\
              & = \frac{9}{13} \times \frac{1}{9} \\ 	 
              & = \frac{1}{13}   	\qedhere
            \end{align*}
          \end{minipage}
        \end{solution}	
      \end{minipage}
    \item 
      \begin{minipage}[t]{0.25\textwidth}
        $\dfrac{9}{10} \div \left[\dfrac{1}{2} \times\left(\dfrac{6}{5} - \dfrac{1}{3}\right)\right]$
        \begin{solution}
          \begin{minipage}{6em}
            \begin{align*}
              \text{原式}&	= \frac{9}{10} \div \left[\dfrac{1}{2} \times \dfrac{13}{15} \right]\\
              & = \frac{9}{10}  \div \frac{13}{30}\\
              & = \frac{27}{13} \qedhere 	    	
            \end{align*}
          \end{minipage}
        \end{solution}	   
      \end{minipage}
  \end{choices} 
\end{problem}

\end{document}

\documentclass{exam-zh}
\examsetup{
  solution={
    show-solution = true
  }
}

\begin{document}


\begin{choices}[label = {}]
  \item
    \begin{minipage}{0.4\textwidth}
      $ 1 + 2 + 3 = $ 
      \begin{solution}[blank-type = none]
        \begin{align*}
          1 + 2 &= 3\\
          3 + 3 &= 6 \qedhere
        \end{align*}
      \end{solution}
    \end{minipage}
  \item
    \begin{minipage}{0.4\textwidth}
      $ 1 + 2 + 3 = $ 
      \begin{solution}[blank-type = none]
        \begin{align*}
          1 + 2 &= 3\\
          3 + 3 &= 6 \qedhere
        \end{align*}
      \end{solution}
    \end{minipage}
\end{choices}

\begin{choices}[label = {}]
  \item
    \begin{minipage}{8em}
      $ 1 + 2 + 3 = $ 
      \begin{solution}[blank-type = none]
        \begin{align*}
          1 + 2 &= 3\\
          3 + 3 &= 6 \qedhere
        \end{align*}
      \end{solution}
    \end{minipage}
  \item
    \begin{minipage}{8em}
      $ 1 + 2 + 3 = $ 
      \begin{solution}[blank-type = none]
        \begin{align*}
          1 + 2 &= 3\\
          3 + 3 &= 6 \qedhere
        \end{align*}
      \end{solution}
    \end{minipage}
  \item
    \begin{minipage}{8em}
      $ 1 + 2 + 3 = $ 
      \begin{solution}[blank-type = none]
        \begin{align*}
          1 + 2 &= 3\\
          3 + 3 &= 6 \qedhere
        \end{align*}
      \end{solution}
    \end{minipage}
\end{choices}

\end{document}